This invention relates generally to communications in which total available spectral bandwidth is larger than that needed for communication and, more particularly, to optimization of communication systems in which communicating devices are allowed to change carriers in order to make use of excess available bandwidth. Furthermore, the invention relates to wireless channels that are impacted by multi-path conditions giving rise to large signal strength variations in the frequency domain.
In wireless communication systems, data to be communicated is typically transmitted in bursts on a carrier whose characteristics may vary over time. In other words, a first burst of data might be transmitted over the carrier while the carrier has very good performance that allows the first burst of data to be received correctly, while, as a second burst of data is transmitted on the carrier, the performance of the carrier might have worsened such that the second burst of data is not received correctly. This problem can be explained by the fact that the channel includes multiple paths between the transmitter and the receiver; therefore, even a small movement by either a transmitter or a receiver can affect whether these multiple paths combine constructively or destructively.
If a rate of change of the performance of a carrier is relatively great in comparison to a data rate on the carrier, the problem of varying carrier performance can be solved using coding and interleaving, in which carrier performance variations are averaged so that the carrier's performance depends on average carrier conditions rather than on worst-case carrier conditions. However, if the carrier's performance varies relatively slowly and/or if the data rate is relatively great, this approach is not feasible because the number of symbols needed in an interleaver is too large. In such situations, an entire packet could be received during a period in which the carrier's performance is poor.
Multi-path phenomena are frequency-selective; therefore, if performance of a first carrier having a first frequency is poor, performance of a second carrier having a second frequency is often better, especially if the second frequency is not too close to the first frequency. The coherence bandwidth is a measure of how far apart the two frequencies must be in order for the two carriers to be uncorrelated.
Reference is now made to FIG. 1, wherein there is shown a graph representing an exemplary channel frequency response 104 of a radio channel measured from 2400 MHz to 2440 MHz. Graph 100 shows frequency in Megahertz (MHz) on an x-axis and on a y-axis, signal strength is plotted in decibels (dB) (i.e., 20 log [abs (H(f))]). It only shows the part from 2400 MHz to 2440 MHz. In practice, the entire unlicensed 2.4 GHz Industrial, Scientific, and Medical (ISM) band from 2400 MHz to 2483.5 MHz is available for radio communication applications. For the sake of clarity, only the lower part is shown here. The spectral representation of the signal is shown as the transmit spectrum 102. As an example in FIG. 1, the transmit spectrum 102 is placed at 2415 MHz. In this area, the channel frequency response 104 is fairly flat and shows low attenuation. The transmit spectrum 102 would have worse performance when placed at 2406 MHz, for example, where there is a fading dip in the spectrum. The radio channel of FIG. 1 has a coherence bandwidth of about 10 MHz.
One way of communicating over a frequency-selective carrier is by means of frequency hopping (FH), which is used, for example, in the BLUETOOTH® wireless technology system. See, for example, J. C. Haartsen, “The Bluetooth radio system,” IEEE Personal Communications, Vol. 7, No. 1, February 2000. In the BLUETOOTH® wireless technology system, which is an ad-hoc system that operates in the unlicensed ISM band at 2.4 GHz, one of the reasons for employing FH over 79 carriers, each spaced apart from one another by 1-MHz, is to avoid transmitting on a single carrier that could be strongly attenuated for a long time period due to multi-path fading. Another reason for using FH is to have a system that is robust to interference from other users as well as from other impairments.
Frequency hopping is a way of averaging quality of the total available bandwidth and, in situations in which the carrier performance changes rapidly, FH often provides best-case real-world performance. However, in situations in which a portion of the bandwidth changes slowly, it would be desirable to further improve performance. For example, if a part of the bandwidth is disturbed by an almost static interferer, this part of the bandwidth should typically be avoided. A static interferer could, for example, be a switched-on microwave oven, since many microwave ovens use part of the ISM band.
Carriers that are operating in a part of the bandwidth that is disturbed by almost-static interferer(s) should be avoided. A procedure for selecting suitable carriers that are not affected by the almost-static interferer(s) would be desirable.
In addition, it is clear from FIG. 1 that regions in the frequency spectrum exist where the signal strength is low or is varying considerably. These regions should be avoided as well. Destructive multi-path conditions give rise to additional attenuation in specific frequency regions. In order to achieve acceptable performance, the output power of the transmitter needs to be increased to compensate for the additional loss. Moreover, if the signal bandwidth 106 of the transmit spectrum 102 is wide with respect to the variations in the channel frequency response (i.e. the signal bandwidth encompasses several fading dips), severe distortion of the signal results. This is because the delay difference between the different multi-paths is so large with respect to the symbol time, that symbols arriving from different paths interfere with one another. This self-interference is also referred to as Inter Symbol Interference or ISI.
Crucial for a system that dynamically determines which carrier to use is channel assessment. In the channel assessment scheme, the transceiver has to judge whether the considered carrier has both low interference conditions and a flat fading response. The latter becomes more important when signals with a wider signal bandwidth and with more complex modulation schemes are used. Interference can be measured by merely scanning a specific frequency band. No transmission is required from the associated transmitter. In contrast, the channel response can only be measured when the transmitter sends a signal with known characteristics.
In U.S. patent application Ser. No. 09/894,050 (henceforth, “the '050 application”), filed on Jun. 28, 2001 and entitled “Method and System for Dynamic Carrier Selection”, and published as International Publication Number WO 02/37692 A2 on May 10, 2002, describes a system for applying dynamic carrier selection in a high-speed mode of BLUETOOTH® wireless technology. In the basic mode providing data rates up to 3 Mb/s, the BLUETOOTH® technology applies frequency hopping; in the high-speed (HS) mode providing data rates up to 12 Mb/s, the BLUETOOTH® technology applies dynamic carrier selection (DCS) of a static carrier which can be placed at 77 different positions in the 2.4 GHz ISM band. In the '050 application, a method is presented in which the receiver, operating in HS mode, regularly scans the ISM band to check for interference.
Yet another U.S. patent application, U.S. patent application Ser. No. 10/893,305 , filed on Jul. 19, 2004, describes a method of finding parts in the channel frequency response 104 which are relatively flat. A flat spectrum prevents frequency-selective fading and allows the system to change to more complicated modulation schemes which provide higher data rates but are less tolerant of distortion. Also in U.S. patent application Ser. No. 10/893,305, a method is described applying a combined DCS and link adaptation scheme.
The afore-mentioned techniques work fine for avoiding interference, and provide flat channel conditions in fairly stable environments. When the transceivers are stationary, the channel frequency response does not vary over time. However, when the transceivers move, the multi-path environment constantly changes. As a result, the channel frequency response 104 also constantly changes. Measurements become less reliable and more often the system must switch to a new carrier frequency.
There is accordingly a need for a method and system that can provide reliable communications in a multi-path environment which is rapidly changing.